In coils for electronic or electrical equipment such as inverter-related equipment, for example, high-speed switching devices, inverter motors and transformers, electrical insulated wires (insulated wires), which are enameled wires, are mainly used as magnet wires. It is required for insulated wires to minimize partial discharge deterioration, which is attributable to inverter surge. In order to prevent the partial discharge deterioration, studies have been conducted on methods of increasing a partial discharge inception voltage, and one of them includes a method of increasing a thickness of an insulation covering of the insulated wire. However, in the case where the thickness of the insulation covering is increased, adhesion with the conductor is reduced.
Therefore, attempts have been made on pursing high value-added properties based on a newly provided covering resin layer, in addition to increasing the partial discharge inception voltage, by providing, as a multilayer insulation covering, the covering resin layer formed of a resin having properties different from those of an enamel-baked layer outside the enamel-baked layer. For example, Patent Literature 1 proposes “inverter surge-resistant insulated wire capable of realizing achievement of a thick film of an insulating layer for increasing partial discharge inception voltage without decreasing adhesive force between a conductor of an insulated wire and an enameled layer” by providing a multilayer insulation covering. Moreover, Patent Literature 2 proposes an insulated wire having a high partial discharge inception voltage, and excellent abrasion resistance of a coating, and the like by providing a multilayer insulation covering.